1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and more particularly to a fixing device included in an image forming apparatus for fixing a toner image carried on a sheet or recording medium by using a belt.
2. Description of the Background Art
An image forming apparatus includes a fixing device for fixing a toner image carried on a sheet or recording medium with heat. One of the conventional fixing devices is configured to convey a sheet with a pair of rollers while causing one of the rollers to heat a toner carried on the sheet. Another conventional fixing device includes a roller and a belt so positioned as to form a nip there between, as disclosed in, e.g., Japanese Patent Laid-Open Publication No. 10-307496. Generally, a belt can be made smaller in volume and therefore in thermal capacity than a roller and can be warmed up in a short period of time. Warm-up time is therefore shorter with the combination of a roller and a belt than with only a roller.
However, a problem with the fixing device of the type using a belt is that the belt lacks flatness, i.e., it is apt to wave or slacken. If the belt waves or slackens, then it is likely that a toner image carried on a sheet is shifted or that toner is transferred from the sheet to the belt, resulting in a defective image. Another problem is that after the toner image has been fixed on the sheet, the sheet cannot be surely separated from the belt.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-160405, 10-307493, 11-2979 and 11-45025.
It is an object of the present invention to provide a fixing device of the type using a belt as one of members for forming a nip and capable of obviating defective images ascribable to the waving or the slackening of the belt, and an image forming apparatus including the same.
It is another object of the present invention to provide a fixing device capable of surely separating a sheet after the fixation of a toner image carried on the sheet, and an image forming apparatus including the same.
A fixing device included in an image forming apparatus fixing a toner image carried on a sheet of the present invention includes an endless belt passed over a plurality of members for conveying the sheet in contact with the toner image carried thereon. A pressing member faces the plurality of members with the intermediary of the belt. A first member, which is one of the plurality of members, constitutes a heating member for heating the belt. A second member, which is another one of the plurality of members, faces the pressing member with the intermediary of the belt. A third member, which is another one of the plurality of members, constitutes a stationary member facing the pressing member with the intermediary of the belt. The pressing member and first member and the pressing member and second member respectively form a first and a second nip therebetween. The toner image on the sheet is fixed by being sequentially conveyed via the first and second nips.